Sri Lanka has declared a state of emergency after Foreign Minister Lakshman Kadirgamar was assassinated in a gun attack in the capital, Colombo.

President Chandrika Kumaratunga has appealed for "calm and restraint".

The killing was blamed by a senior police officer on the Tamil Tiger rebel group, which has been observing a ceasefire since 2002.

But the rebels have denied killing the foreign minister, a Tamil who was a fierce critic of the group.

"We strongly condemn this attempt to put the blame on us and we strongly deny any involvement in this assassination," SP Tamilselvan, who heads the rebels' political wing, told the BBC's Tamil Service.

Correspondents say that the rebels have rarely accepted responsibility for many of the attacks they have carried out in the past.

The Norwegian monitors overseeing the truce have warned that the killing could put the ceasefire at risk.

He waged a relentless war against terrorism in all its forms despite continuous threats to his life

Mr Kadirgamar had considered himself to be a potential target of the group, reports said.

Vilja Kutvonen, spokeswoman for Nordic Sri Lanka Monitoring Mission, told Reuters the minister's death was a big blow to the peace process.

She said: "It's likely to have serious consequences. It puts the whole ceasefire under risk."

Peace efforts

The truce has recently been under growing strain, amid rebel claims that the government was continuing to conduct a covert war against them.

This senseless murder was a vicious act of terror

Condoleezza RiceUS Secretary of State

He was appointed foreign minister in April 2004, but had previously held the position from 1994 to 2001.

A lawyer by training, Mr Kadirgamar was seen as a tough opponent of the Tamil Tigers. He played a key role in getting the group listed as a banned militant organisation in the US and the UK.

Neighbouring India condemned what the foreign ministry described as a "terrorist crime" and offered its full support. US Secretary of State Condoleezza Rice, who last met Mr Kadirgamar in June, said those responsible must be brought to justice.

She praised Mr Kadirgamar as a man of "dignity, honour and integrity, who devoted his life to bringing peace to Sri Lanka".

"Together, we must honour his memory by re-dedicating ourselves to peace and ensuring that the ceasefire remains in force," she added.

UN Secretary General Kofi Annan said in a statement quoted by the AFP news agency:

"Sri Lanka has lost a deeply respected statesman dedicated to peace and national unity."